Although metal roofs are known for their durability and long lives, they do corrode over time and require periodic replacement. Replacing a metal roof is a major undertaking that typically requires a major investment in terms of manpower and time. Traditionally, an old roof was painstakingly torn off a building and hauled away before a new roof was added, exposing the occupants of the affected building to the weather. To reduce time and cost, roofers have more recently taken to leaving an old roof in place and installing a new roof directly atop the old roof.
Different manufacturers provide metal roofing materials with different surface contours or corrugations. Hundreds of profiles are currently available and are variously touted for their abilities to shed precipitation and to withstand deformation under load. Since it is difficult to match the profiles of a new roofing material to an old one, some roofers attach horizontal rows of boards to the top of an old metal roof prior to installing a new roof. These boards position the new roofing materials a fixed distance above the old ones so that surface contours of the old and new roofing materials do not have to match. Unfortunately, wooden boards tend to warp over time causing new roofs to buckle and leak. Additionally, because the boards are substantially rigid, stepping on a new roof at any point away from the boards can cause a permanent bend or crease in the new roofing materials.